N-methyl-D-aspartate receptor (NMDA receptor or NMDAR) is a glutamate receptor and ion channel in nerve cells. Activation of NMDAR allows positively charged ions to flow through the membrane. NMDA receptors play a role in physiological processes affecting memory and mood. See e.g., Nicholls et al., Neuron, 2008, 58(1):104-17. Binding of an agonist (such as N-methyl-D-aspartate or glutamate) and a co-agonist (such as glycine or D-serine) is required for NMDAR activation.
Agents that modulate NMDA receptors have been reported to be useful in a variety of therapeutic applications. For example, memantine is used to treat Alzheimer's disease and Lewy Body Dementia. However, treatment with NMDA modulators can have side effects such as sedation and hallucinations.
Anti-NMDAR encephalitis is an autoimmune encephalitis characterized by the presence of antibodies against synaptic NMDAR. Anti-NMDAR encephalitis (also known as NMDA receptor antibody encephalitis or NMDAR encephalitis) has become the most common and best characterized antibody-defined autoimmune neuronal disorder. The encephalitis associated with antibodies against NMDAR predominantly affects children and young adults, occurs with or without tumor association, responds to treatment, but can relapse. The exact incidence of anti-NMDAR encephalitis is unknown. Due to the rareness of the syndrome and the varied clinical presentations, anti-NMDAR syndrome may be misdiagnosed and under-recognized.
Schizophrenia is a chronic and devastating neuropsychiatric disorder that is ranked as a leading cause of disability worldwide. The disease afflicts nearly 1% of the world's population, affecting both men and women equally, and striking all ethnic and socioeconomic groups with a similar level of prevalence. The illness is characterized by multiple symptoms that are categorized into three clusters known as positive symptoms (hallucinations and delusional behaviors), negative symptoms (anhedonia, social withdrawal and apathy), and cognitive dysfunction (diminished capacity for learning, memory, and executive function). Currently available antipsychotic drugs exhibit efficacy for positive symptoms, but have been limited in their capacity to treat negative symptoms and cognitive deficits.
D-serine occurs naturally in the human body, although in much smaller amounts than L-serine. Only L-serine is found in proteins.
D-serine is an agonist of NMDA receptors. Academic studies have demonstrated that oral dosing of D-serine can result in dose-dependent improvement in positive, negative, and cognitive symptoms in schizophrenic patients when added to D2 antipsychotics (antipsychotic drugs that bind to and inhibit or block the activation of dopamine D2 receptors). However, preclinical studies have demonstrated that administration of D-serine can cause nephrotoxicity in rats. In addition, in some patients who received high doses of D-serine, clinical findings suggesting renal impairment were observed. As a result, the clinical development of D-serine has historically been limited.
There remains a need for improved treatments for NMDAR encephalitis and neurological conditions mediated by the NMDAR, including schizophrenia.